Verticillium Wilt
Causal Agents
Verticillium wilt in cucurbits is caused by the fungus Verticillium dahliae.
Symptoms
Symptoms of Verticillium wilt are first seen on the lower foliage that turns chlorotic and necrotic (dead areas). In some hosts, the foliar symptoms are distinct. Decreased photosynthesis reduces the overall fruit quality. In severe infections, plants fully wilt and die with discolored vascular tissue.
Disease Cycle
Verticillium dahliae is potentially found in most soil types across Utah. These fungi can survive several years in the soil as small hard balls of mycelium called microsclerotia. The ideal temperature for V. dahliae infection is within 70 °F to 85 °F. Under these temperatures with wet soils, microsclerotia germinate, and the fungus infects plants through intact or damaged roots. From there, hyphae invade the xylem tissue and produce spores that are transported upward throughout the vascular tissue. Eventually, the plant wilts and dies, and the fungus produces microsclerotia that are released into the soil when the plant matter decays.
Management
Since infected plants cannot be treated (and should be removed), it is important to use cultural control practices to prevent introducing of the disease and improve soil health.
Preventing introduction:
- Rotate between different crops to prevent wilt fungi from building up in the soil.
- Source disease-free seed. Though uncommon, uncertified disease-free seeds and tubers may harbor Verticillium spp. or Fusarium spp.
- When starting seeds and caring for transplants in a greenhouse, sanitation is critical in preventing pathogens like Fusarium and Verticillium spp. that may eventually get transferred to the field. Sterilize pots and trays by soaking in a 10% to 15% bleach solution for half an hour to an hour or use quaternary ammonium compounds. Pots and trays need to be rinsed well with water afterward to avoid phytotoxicity from residue. Disinfect greenhouse benches and other supplies. If infections do occur, dispose of seedlings along with the potting mix as it may be harboring the pathogens.
- After working in fields, ensure workers clean off soil from their shoes, tools, and mechanical equipment when moving from one field to another.
Where the disease has been diagnosed:
- Plant vegetable cultivars with resistance to Fusarium and Verticillium wilts. Cantaloupes have many varieties available. Note that some cultivars will only have resistance to one of multiple races of the pathogen, and this will be noted on the seed packets.
- Remove all plant residues at the end of the season to reduce the population of overwintering fungal structures.
- In commercial production, soil fumigation using high concentrations of the fumigants metam sodium, and chloropicrin can reduce microsclerotia populations in the soil. However, this treatment can be hazardous to overall soil health. This method is also not economically viable for Utah farmers.
- Research shows cover crops in the mustard family have some biofumigation properties due to the high presence of glucosinolates. Research on biofumigation is ongoing.